Comment 21 for bug 841409

For those who do not wish to wait for a patch to fix this, you can add the following line to the [Default Applications] section of the file $HOME/.local/share/applications/mimeapps.list:

text/calendar=thunderbird.desktop

I also added the same line to the [Added Associations] section, but I am not sure if that is necessary.

Once you do this, Thunderbird becomes a recommended application for that file type, and shows up in the list. If the Google Calendar app, or any other calendar app has a .desktop file, you can do the same for that app.